Nitrogen oxides (NOx), which can have an impact on the environment, are included in exhaust gas emitted from internal combustion engines such as diesel engines. Conventionally, as one aspect of exhaust purification apparatus used for purifying these NOx, there are known SCR (Selective Catalytic Reduction) systems where a selective reduction catalyst is disposed in an exhaust pipe and where reduction and purification of NOx is performed using an additive in this selective reduction catalyst. These SCR systems deliver an additive such as a urea solution or unburned fuel to the exhaust pipe on the upstream side of the selective reduction catalyst and selectively reduce and purify NOx in the exhaust gas in the selective reduction catalyst.
Further, in addition to SCR systems using a selective reduction catalyst, there are also known exhaust purification apparatus using a NOx storage catalyst that stores NOx in the exhaust gas in a state where the air-fuel ratio of the exhaust gas is lean and releases the NOx when the air-fuel ratio switches to a rich state. As one aspect of these exhaust purification apparatus, there is an exhaust purification apparatus that directly adds unburned fuel to the upstream side of the NOx storage catalyst to cause the NOx stored in the NOx storage catalyst to be released and uses a hydrocarbon (HC) included in the unburned fuel to perform reduction and purification of the NOx.
Representative examples of devices that deliver the urea solution or unburned fuel used in these exhaust purification apparatus include injection-type additive delivery devices that pressure-feed the additive with a pump and deliver the additive into the exhaust pipe with an injection valve connected to the exhaust pipe. Further, among devices that deliver the urea solution or the like, there are also air assist-type additive delivery devices that use high-pressure air to atomize the urea solution or the like inside a mixing chamber and spray the urea solution into the exhaust pipe via a nozzle connected to the exhaust pipe.
Such additive delivery devices are equipped with a storage tank that stores the additive such as the urea solution or unburned fuel. The additive delivery devices cannot perform delivery control of the additive with good precision when the additive stored inside this storage tank solidifies, so some additive delivery devices are equipped with a temperature sensor inside the storage tank and can detect the temperature of the additive. For example, as an additive delivery device using a urea solution as the additive, there has been disclosed an additive delivery device equipped with an assembly for delivering the urea solution comprising a quality sensor, a temperature sensor, a level sensor and a pump in a storage container (a storage tank) (see patent document
Patent Document 1: JP-T-2002-527660 (claim 12, paragraph [0038])